1. Field of the Invention
The invention relates to a piston assembly, and more particularly to a piston assembly for a pump mechanism that delivers a sterile fluid for medical treatment.
2. Description of Related Art
The pressurized delivery of fluids to a patient involves numerous challenges and concerns. Doctors and perfusionist often face issues relating to: delivery of a precise volumetric flow or a precise ratio of two fluids; preserving the sterility of the fluid throughout the fluid's journey through the pumping mechanism; and, if the fluid is blood or a blood-fluid mixture, the infusion of damaged blood, such damage occurring through the features inherent to the most widely used pumping systems.
A variety of systems exist for the purpose of pumping fluids for medical infusion. Two of the more widely used classes of fluid pumps include roller pumps and centrifugal pumps. A roller pump uses a rotating, variably-occlusive roller to systematically push fluid through tubing by compressing the tubing against a track that partially surrounds the rotating roller. A centrifugal pump possess a turbine, to which the fluid being pumped must come in contact, that pushes the fluid along a fluid path. Both pumps possess distinct features that can damage blood or the blood constituent of a blood-fluid mixture. Hemolysis, or the physical destruction of red blood cells, can occur through either the compressive forces applied by the roller of the roller pump, or the shear forces created by the turbine of the centrifugal pump. Moreover, neither of these pump systems provide a means for delivering consistent, precise volumetric quantities or allowing the consistent mixture of multiple fluids in precise ratios. Other pumping means which exist, but are not as prevalent as the aforementioned, include a "rolling diaphragm," as disclosed in U.S. Pat. No. 4,391,600 (Archibald), and a "petal assembly," as disclosed in U.S. Pat. 4,657,490 (Abbott).
U.S. Pat. No. 4,391,600 discloses a non-pulsating I.V. pump that receives a disposable fluid cassette. The '600 Patent features cylindrical pump chambers that accommodate cylindrical pistons, such pistons having an outer diameter that is less than the inner diameter of the pump chambers. The fluid cassette possesses cylindrical pumping chambers that fit within the pump's chambers. During operation, the cylindrical chambers of the disposable fluid cassette must "rolls" over itself within the space between the piston and the pump chamber as the piston advances. The result of such rolling is (i) the continuous inducement of varying stresses at the supported-unsupported interface of the cassette; (ii) fluid and/or air may be trapped in the unsupported portion of the cassette during the advancement of the piston, such compromising the volumetric accuracy of the expelled fluid; and (iii) the flexible material of the cassette is required to be pinched and worked throughout the fluid delivery procedure. Importantly, the first and third of these can act on material imperfections or forming imperfections to compromise the integrity of the disposable fluid cassette.
U.S. Pat. No. 4,657,490 discloses an infusion pump that also uses a disposable cassette. The '490 infusion pump features a petal assembly which acts on the cassette. Specifically, the petal assembly includes a central hub surrounded by a plurality of rigid, petal shaped sections. Each petal section spans between the central hub and the fixed housing with surrounds the petal assembly. In operation, the petals form a movable pressing surface. As may be understood, the stroke of the petal assembly is limited by the motion of the petals. Notwithstanding, the shallow angles inherent to the petal assembly are necessary to maintain a constant surface area to support the cassette. As a consequence, however, the shallow petal angles limit the petal assembly's volumetric pumping capacity. Nevertheless, if the petal assembly was altered to move away from the concept of providing a constant supporting surface, the petal assembly becomes incapable of ensuring volumetric accuracy as the resulting pumping chamber would likely facilitate trapped fluids and/or air.
In light of present pumping methods, a need remains for a simple piston assembly that allows for the safe displacement of a relatively significant volume of fluid while also being capable of maintaining consistent volumetric accuracy. Further, such piston assembly should facilitate use of a disposable fluid cassette, wherein the piston assembly provides a constant supporting surface area throughout the pumping motion to prevent damage to the cassette by stretching or allowing the cassette to become pinched or bound during the operation.